toggleSetProblem.js

// https://jsbin.com/lagobuzoxu/1/edit?html,js,output

let state = []

const handleToggle = value => {
  state = state.includes(value)
    ? R.without([value], state)
    : [...state, value] 
}

const getToggleSet = R.symmetricDifference

const reverseStateTo = oldState => () => {
  getToggleSet(state, oldState).forEach(value => (
    handleToggle(value)
  ))
}

QUnit.module('reverseStateTo')

QUnit.test('it reverts to initial when more items were added', assert => {
  state = ['a', 'b', 'c']
  reverseStateTo(['a', 'b'])()
  
  assert.deepEqual(state,  ['a', 'b'])
})

QUnit.test('it reverts to initial when items were remvoved', assert => {
  state = ['a', 'b']
  reverseStateTo(['a', 'b', 'c'])()
  
  assert.deepEqual(state,  ['a', 'b', 'c'])
})